PURPOSE: A feed efficiency improver for livestock, comprising specific substances without remaining and accumulating in animal bodies, and producing undesirable effects, e.g. resistant bacteria, on human bodies when used for foods, as active constituents.

CONSTITUTION: A feed efficiency improver comprising antibiotic salinomycin, obtained from a culture of SY-1 bacteria, Streptomyces albus Waksman and Henrici No.80614 (FERM-P No.419), having an antibacterial, antiprotozoal, and preventing effects on poultry and livestock against coccidioses, as an active constituent. The improver is administered to livestock, e.g. ruminants, such as cattle, in a suitable concentration e.g. 1W100ppm for cattle, of the active constituent.